A global period of climate change 42,000 years ago was the result of a reversal in Earth's magnetic field, new research has found. 

According to radiocarbon analysis of preserved ancient tree rings, climate change, mass extinctions and even changes in human behavior over several centuries can be directly linked to the earth's magnetic field changing its polarity at that time.

The research team named this period of dramatic change the "Adams Transient Geomagnetic Event" after science fiction writer Douglas Adams, who declared the number 42 to be the ultimate answer to life, the universe and everything.

"For the first time in history, we have been able to precisely date the timing and environmental impact of the last magnetic pole reversal," said scientist Chris Turney of the University of New South Wales in Australia." 

The discovery was made possible by ancient New Zealand kauri trees, preserved in sediments for more than 40,000 years. Using the ancient trees, we were able to measure and date the peak in atmospheric radiocarbon levels caused by the collapse of the Earth's magnetic field.”

This last period of magnetic reversal is known as a Laschamp event and we call it a geomagnetic excursion. The planet's magnetic poles briefly switch places during this event before returning to their original position. It is one of the best-studied events of changing the Earth's magnetic field, recorded by ferromagnetic minerals.

It happened about 41,000 years ago and lasted about 800 years. However, what exact impact this event had on life on the planet is unclear. So when scientists in 2019 discovered an ancient kauri tree (Agathis australis) that also grew during this period, they jumped at the chance to learn more.
Trees record atmospheric activity in their tree rings. In particular, carbon-14, or radiocarbon, can reveal a lot of information about magnetic activity.

Radiocarbon is only found in trace amounts on Earth, compared to other naturally occurring isotopes of carbon. It is created in the upper layers of the atmosphere due to cosmic radiation. When cosmic rays enter the atmosphere, they interact with local nitrogen atoms and start a nuclear reaction that produces radioactive carbon. Because cosmic rays come in all the time, Earth receives a steady supply of radiocarbon.

So the spike in radiocarbon in the rings of preserved tree wood tells us that the Earth was exposed to more radiocarbon that year. When the Earth's magnetic field is weakened, as it was during the Laschamp event, more cosmic rays penetrate the atmosphere, producing more radiocarbon.

Because of this, scientists have previously been able to determine that the Earth's magnetic field has weakened to about 28 percent of its normal strength during these 800 years. However, the kauri tree allowed the research team to study the years leading up to the event. They found that the Adams event occurred about 42,200 years ago, and the magnetic field was at its minimum before that.

"During the Adams event, Earth's magnetic field dropped to just 0-6 percent," Turney explained. " We basically had no magnetic field at all – our shield against cosmic rays was completely gone."

During this time, the Sun's magnetic field also weakened several times as the Sun also experienced a magnetic reversal, which is part of its regular cycle. During these periods, there is less sunspot activity and fewer eruptions. The Sun's magnetic field provides the Earth with some degree of protection from cosmic rays, so during these solar minima, the strength of cosmic rays increased again.

A weakened magnetic field likely caused substantial changes in Earth's atmospheric ozone with dramatic consequences, including electrical storms, spectacular auroras, and climate change around the world.

"Unfiltered radiation from space broke down the air particles in Earth's atmosphere, stripped electrons and emitted light -- a process we call ionization," Turney said. "Ionized air warmed the ozone layer, triggering a wave of climate change around the world."

This is consistent with climate and environmental changes from this time seen in other records from around the world, such as the mysterious extinction of Australian megafauna.

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